Generally, exhaust gas flowing out from an engine through an exhaust manifold is driven into a catalytic converter mounted at an exhaust pipe, and is purified therein. After that, noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matter (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification catalyst (DeNOx catalyst) is one type of such a catalytic converter, and purifies nitrogen oxide (NOx) contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbon (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced in the DeNOx catalyst through oxidation-reduction reactions with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst has been used as such a DeNOx catalyst. The LNT catalyst adsorbs the NOx contained in the exhaust gas when an air/fuel ratio is lean, and releases the adsorbed NOx and reduces the released nitrogen oxide and the nitrogen oxide contained in the exhaust gas when the air/fuel ratio is rich.
If a temperature of the exhaust gas, however, is high (e.g., the temperature of the exhaust gas is higher than 400° C.), the LNT cannot purify the nitrogen oxide contained in the exhaust gas. Particularly, if a particulate filter for trapping particulate matter (PM) contained in the exhaust gas is regenerated or sulfur poisoning the LNT is removed, the temperature of the exhaust gas increases. Therefore, the nitrogen oxide contained in the exhaust gas is not purified, but is exhausted to the exterior of the vehicle.
Recently, as exhaust regulations become tightened in most countries, a system of purifying an exhaust gas provided with an additional DeNOx catalyst (e.g., selective catalytic reduction (SCR) catalyst) together with the LNT has been developed. Since operating regions of the LNT and the SCR catalyst overlap with each other, however, it is hard to achieve a target purifying ratio. According to experiments, if the LNT and the SCR catalyst are not controlled by a specific strategy, a purifying ratio is increased by 10%-20% compared with a purifying ratio of any one of the LNT and the SCR catalyst. Therefore, tightened exhaust regulations cannot be met. Therefore, optimum control of the LNT and the SCR catalyst are necessary.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.